Fuel cell stack clamping structure and solid oxide fuel cell stack

ABSTRACT

A clamping structure for a planar solid oxide fuel cell stack comprising a flexible sheet and a rigid, thermally insulating end block, the flexible sheet being capable of bending into a primarily convex shape, the rigid, thermally insulating end block shaped as a rectangular base with a planar surface and an opposing surface that is primarily convex in shape, the flexible sheet being placed adjacent to the opposing surface of the rigid, thermally insulating end block, the flexible sheet thereby bending to obtain a shape that is primarily convex. The invention also relates to a solid oxide fuel cell stack and a process for the compression of the stack.

This is a continuation-in-part of application Ser. No. 11/698,073, nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a clamping structure for a solid oxide fuelcell stack. More particularly, the invention relates to a planar solidoxide fuel cell stack which is compressed using a clamping structurewhich includes a four-sided planar flexible sheet.

2. Description of the Related Art

A planar solid oxide fuel cell (SOFC) stack consists of a repeatedsequence of solid oxide fuel cells across which an electrical voltage iscreated alternating with interconnects.

The stack typically includes 5 to 200 fuel cells and consists of asequence of fuel cells comprising an anode, a cathode and a solid oxideelectrolyte each fuel cell alternating with the interconnect. The fuelcells are provided with fuel and oxidant by a manifold system via aninternal channel system. Fuel and oxidant are distributed from layer tolayer in the fuel cell stack by a channel system. During operation, anelectrochemical voltage is created across the individual fuel cells. Theinterconnect serves to introduce oxidant and fuel to the fuel cells inseparate channels and to collect electrons from one fuel cell andtransmit and deliver them to an adjacent fuel cell.

The walls of the internal channel system must be gas tight in order toavoid leakages of gas to the external surrounding or untimely mixing ofoxidant and fuel. This is ensured by using a sealing material of forinstance glass, and/or by providing an intimate and direct bondingbetween the fuel cell and interconnect on the available sealingsurfaces.

The gas tight behaviour and the desired electrical contact between thefuel cells and interconnects are ensured in a SOFC stack by pressing thefuel cells and interconnects together with a well-defined compressiveforce using a clamping structure. In some cases the required compressiveforce can be as high as 100 N/cm² across each fuel cell surface duringoperation of the fuel cell stack. The magnitude of the compressive forceis dependent on the actual design of the interconnect and fuel cell andon the gas pressure during operation. The compressive force is providedon the end surfaces of the stack.

A SOFC stack typically operates at temperatures of 600-850° C. Such hightemperatures represent a challenge to the design of the mechanicalclamping structure required to generate compressive forces of such amagnitude.

It is important that the compressive force is exerted on a surface areacorresponding to the surface area of the fuel cells in the stack. Theinner sections of the end surfaces of the stack must be compressed inorder to maintain electrical contact and the peripheries of the endsurfaces must be compressed in order to make the stack gas tight.Conventionally, fuel cells have surface areas of 80-1000 cm² andcompressive forces of up to 100,000 N can be required.

Various types of clamping structures or assemblies are known forinstance assemblies using bands for compressing planar fuel cell stacks.U.S. Pat. No. 5,993,987 discloses a fuel cell stack comprising at leastone band circumscribing end plates and interposed electrochemical fuelcells. A resilient member cooperating with the band urges the end platestowards each other thereby applying compressive force to the fuel cellsto promote sealing and electrical contact between the layers forming thefuel cell stack.

US patent application No. 2006093890 discloses a fuel cell stackmaintained in compression by a strap assembly that includes acompressive band extending around the end plates of the fuel cell stack.

Traditional clamping structures are based on the compression of ametallic planar end plate flange placed at either end surface of theSOFC stack and extending beyond the surface area defined by the fuelcells in the stack. The two end plate flanges are connected to eachother at their periphery external to the fuel cells by a clampingstructure of tie-rods, pipe sections, springs and nuts for creating acompressive force in the stack.

The forces experienced in the tie-rods can be established with the aidof the elasticity of the tie-rods using disc springs, coil springs, gassprings or using pneumatic cylinders or hydraulic cylinders.

SOFC stacks typically operate at temperatures of 600-850° C. At thistemperature most metallic materials when subjected to mechanical stresswill creep with time. It is therefore advantageous to maintain themetallic sections that experience mechanical stress at as low atemperature as possible.

The tie-rods are typically inserted through the two planar end plateflanges, thereafter through pipe sections of a specified lengthextending beyond the SOFC stack and through springs placed at the endsof the pipe sections. The pipe sections function as spacers fordistancing the springs from the fuel cell stack such that the springsare maintained at a less severe operating temperature than the hightemperature experienced during operation of the stack. Nuts positionedafter the springs are used to assemble these components and thereby toadjust the compressive force on the SOFC stack.

During operation of the SOFC stack the tie-rods are at a temperatureapproximately equivalent to the stacks operation temperature. Thetension created thereby in the tie-rods results in a tendency of thetie-rods to creep.

During operation of the SOFC stack the planar end plate flanges are alsosubjected to mechanical tension during influence of the forces from boththe tie-rods in the clamping system and the stack causing creep of theplanar flanges. The planar flanges therefore tend to become convex inform.

In an alternative clamping structure the tie-rods and the planar endplate flanges are during operation at a much lower temperature than theSOFC stack's operation temperature. This is made possible by thermallyinsulating the SOFC stack at the sides of the stack using insulationmaterial. Placing additional insulation material at either end of thestack adjacent to the planar end plates allows a transfer of thecompressive force obtained during clamping through the additionalinsulation material. The tie-rods and the planar end plate flanges canthus experience greater tension before undesirable creep sets in. Thedisadvantage of these types of clamping structures using tie-rods areassociated with the planar end plate flange placed at each end surfaceof the SOFC stack and extending beyond the surface area defined by thefuel cells in the stack. Each planar end plate flange experiences abending force when exposed to the mechanical forces originating from thetie-rods and the stack.

These undesirable effects result in a reduction of the compressive forceon the whole stack or in an uneven distribution of the compressive forceon the stack leading to poorer electrical contact and/or the stackbecomes less gas tight and leakage of gas to the external surroundingscannot be avoided.

The flanges used are therefore of a sizeable thickness, typically 5-20mm, in order to absorb these forces and minimise the deformation of theflanges, while simultaneously preventing gas leakage and loss ofelectrical contact in the stack.

WO patent application No. 2006/012844 discloses a fuel cell stack forsolid oxide fuel cells with a clamping device and a heat insulatingdevice. The heat insulating device is located between the fuel cells andthe clamping device, which has pressure distribution elements in theform of either flat plates that are parallel to each other, ahemi-spherical shell or are semi-cylindrical. The pressure distributionelements ensure that the pressure is distributed uniformly on the entiresurface of the heat insulating elements.

No details are given regarding the construction of the pressuredistribution elements, but it is known in the art to use flat platesthat are of metal. Furthermore the application of hemi-spherical shellsimplies the use of a rigid or hard material shaped in the form of ahemi-sphere.

Generally pressure distribution elements in the form of flat plates aremanufactured from metal. Pressure distribution shells or cylinders ofmetal can be prepared by metal forming processes such as deep drawing,which is a more complicated process than the process used in preparingflat plates.

The economy associated with solid oxide fuel cells is high and there isa constant need for a reduction in the cost of SOFC stacks without anylosses in the chemical and/or physical properties of the various stackcomponents.

Furthermore, there is also a need for solid oxide fuel cell componentsthat show acceptable physical properties while contributing to areduction in weight and/or volume of the stack.

It is an objective of the invention to provide a clamping structure fora planar SOFC stack in which deformation due to uneven distribution ofcompressive forces is avoided during operation of the SOFC stack.

It is a further objective of the invention to provide a planar SOFCstack having reduced weight and volume.

SUMMARY OF THE INVENTION

The invention concerns a clamping structure for a planar solid oxidefuel cell stack comprising a planar, flexible sheet and a rigid,thermally insulating end block, the planar, flexible sheet being capableof bending into a primarily convex shape, the rigid, thermallyinsulating end block shaped as a rectangular base with a planar surfaceand an opposing surface that is primarily convex in shape, the flexiblesheet being placed on the opposing surface of the rigid, thermallyinsulating end block, the flexible sheet thereby bending to obtain ashape that is primarily convex upon exertion of a compressive forceacross each solid oxide fuel cell surface.

The invention also concerns a SOFC stack comprising the clampingstructure wherein the stack comprises one or more planar solid oxidefuel cells interposed between end plates, at least one end plate lyingadjacent to the clamping structure comprising a flexible sheet and athermally insulating end block, the flexible sheet being capable ofbending into a primarily convex shape, the thermally insulating endblock shaped as a rectangular base with a planar surface and an opposingsurface that is primarily convex in shape, the flexible sheet beingplaced adjacent to the opposing surface of the thermally insulating endblock, the flexible sheet thereby bending to obtain a shape that isprimarily convex and the at least one end plate being in contact withthe planar surface of the rectangular base of the thermally insulatingend block upon exertion of a compressive force across each solid oxidefuel cell surface.

The invention also concerns a process for the compression of the solidoxide fuel cell stack comprising interposing one or more planar solidoxide fuel cells between end plates, placing adjacent to at least oneend plate a clamping structure comprising a flexible sheet and athermally insulating end block, the flexible sheet being capable ofbending into a primarily convex shape, the thermally insulating endblock shaped as a rectangular base with a planar surface and an opposingsurface that is primarily convex in shape, placing the flexible sheetadjacent to the opposing surface of the thermally insulating end blockbending the flexible sheet to obtain a shape that is primarily convexand placing the at least one end plate in contact with the planarsurface of the rectangular base of the thermally insulating end blockand exerting a compressive force across each solid oxide fuel cellsurface.

A shape that is primarily convex is defined as a shape that is curvedand rounded outwards. The primarily convex shape can be curved in onedirection only i.e. single curved, or it can be curved in all directionsi.e. double curved. The curve and thus the shape can be smoothly orstepwise rounded outwards. Preferable are shapes curved in one directiononly i.e. single curved shapes.

By flexible is meant the ability to bend easily or flex i.e. non-rigid.

When applying the clamping structure of the invention to a planar SOFCstack the use of planar end plate flanges can be avoided completely.This is advantageous since a lower weight of the SOFC stack is thusobtained.

The clamping structure of the invention is flexible in nature and canaccommodate the forces present in the stack during operation withoutcreating any distortion of the various elements in the stack.

Electrical contact between the various layers in the stack is thusmaintained, the stack remains gas tight and leakage of gas to theexternal surroundings is avoided.

Additionally, the SOFC stack of the invention is smaller than theconventional SOFC stack since the thick planar end plates are avoided.Since the planar end plates are usually made of metal, in their absencethe SOFC stack of the invention is lighter and requires less material ormetal for its fabrication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional dissembled SOFC stack.

FIG. 2 shows the conventional assembled SOFC stack.

FIG. 3 shows a conventional assembled SOFC stack.

FIG. 4 shows a conventional dissembled SOFC stack.

FIG. 5 shows a dissembled SOFC stack of the invention.

FIG. 6 shows an assembled SOFC stack of the invention.

FIGS. 7 a, 7 b, 7 c, 7 d and 7 e show transverse cross-sections ofdifferent geometrical embodiments of the insulating end block.

FIGS. 8 a and 8 b show different geometrical embodiments of the flexiblesheet seen from the top of the stack.

DETAILED DESCRIPTION OF THE INVENTION

The clamping structure of the invention is very simple in structure andcomprises a flexible, planar sheet capable of bending and becomingprimarily convex in shape. It comprises also an insulating end blockproviding thermal insulation and shaped in a convex manner on the outersurface. This primarily convex shape of the insulating end block forcesthe flexible sheet into a convex shape when in contact with theinsulating end block and under compression. The shape of the planar,flexible sheet adapts to the shape of the insulating end block. Theprimarily convex surface of the insulating end block adjacent to theflexible sheet thus fits into the convex flexible sheet.

The insulating end block is positioned directly between the flexiblesheet and one end surface of the SOFC stack. No planar end plate flangeis needed between the SOFC stack and the end block. The surface of theinsulating end block adjacent to the SOFC stack is planar and has asurface area identical to the surface area of the SOFC stack i.e. thetwo surfaces have the same overall dimensions. The SOFC stack can alsobe thermally insulated on its remaining surfaces.

The primarily convex surface of the insulating end block forces theflexible sheet to change shape from planar to convex upon compression.The resulting convex flexible sheet thus curves away from the SOFCstack. Thereby, the mechanical tension in the flexible sheet lies in theplane of the flexible sheet. The flexible sheet does not have towithstand bending forces. This allows the flexible sheet to bedimensioned with a much smaller thickness and it is thereby much lighterin weight than the conventional planar end plate flanges. The forcesbetween the flexible sheet and the insulating end block are thereforedistributed in a manner which ensures that the insulating end block isin compression.

The planar flexible sheet can preferably have a length and a breadth of1-2 times the side length of the solid oxide fuel cells in the stack.When the flexible sheet is bent to obtain a shape that is curved in onedirection only i.e. a single curved shape, then it is preferably made bystamping or laser cutting of a thin metal sheet and thereafter bentalong the curvature of the thermal insulation. The sheet can be so thinthat no tools are needed for the bending, which is not the case if thesheet is to be bent into a shape that is curved in all directions i.e. adouble curved shape. This shape requires deep drawing.

The flexible sheet can be smoothly or stepwise rounded outward. Theprimarily convex shape it attains on bending can be curved in alldirections (i.e. double curved) or curved in one direction only (i.e.single curved). The flexible sheet can be bent into a shape that iscurved in all directions and forms a segment of a sphere e.g. a domedshape. Preferably the flexible sheet is bent to form a primarily convexshape that is curved in one direction only (i.e. single curved). Forexample, the flexible sheet can be bent into a shape that is curved inone direction only and forms a segment of a cylinder e.g. an archedshape.

FIGS. 7 a, 7 b, 7 c, 7 d and 7 e show transverse, i.e. verticalcross-sections of different geometrical shapes of the insulating endblock. In all cases the insulating end block has a rectangular basehaving a planar surface and an opposing surface which is primarilyconvex in shape and can be smoothly or stepwise rounded outwards e.g. itcan be stepped, arched or pyramidal. The different geometricalembodiments of the insulating end block all ensure that the flexiblesheet becomes primarily convex in shape and curves in one directiononly. In FIGS. 7 a, 7 d, 7 e the flexible sheet is smoothly roundedoutwards, while in FIGS. 7 b and 7 c the flexible sheet is stepwiserounded outwards.

When using the clamping structure of the invention deformation leadingto curvature of the fuel cell components is avoided. Thick end plateflanges are also not required, thereby leading to a reduction in thevolume and amount of material required to manufacture the flanges, hencereducing the cost of the fuel cell stack. Ultimately, this leads to areduction in the weight of the SOFC stack, which is desirable.

The clamping structure of the invention allows for gas mani-folding atthe sides of the fuel cell stack. The clamping structure can be used ateither end of the fuel cell stack. In addition insulation can be placedon two opposing sides of the four sides of the fuel cell stack,advantageously leaving two other opposing sides available for placementof inlets and outlets for fuel and air to the fuel cell stack.

The clamping structure can further comprise tie-rods, springs and nutsknown in the art and useful for providing a compressive force whenclamping the SOFC stack. The flexible sheet can therefore be providedwith attachment means at its borders which allow passage of for instancetie-rods used in assembling the SOFC stack.

The presence of the insulating end block allows the flexible sheet toexist at a temperature lower than the stack temperature. The insulatingend block has a preferable thickness of 5-100 mm and a thermalconductivity of 0.01-2.0 W/(mK). The thickness of the insulating endblock, its thermal conductivity and the temperature of the surroundingsdetermine the temperature of the flexible sheet during operation of theSOFC stack. It is an advantage to dimension the insulating end block tovalues which allow the flexible sheet to have a temperature of 100-650°C. during operation.

The size of the flexible sheet is at least the size of the cells in thestack, but due to the curvature one of the two sides will be somewhatlonger, preferably 1-2 times the corresponding cell side length.

The flexible sheet is preferably made of steel. However, other types ofmetal alloys are also useful, for instance alloys based on titanium,aluminium or nickel. A suitable alloy is inconel, which is useful at thehigh temperatures employed during operation of the stack due to its heatresistance properties. The flexible sheet can be in the form of a thinmetallic plate having a thickness of for instance 0.05-5 mm, wherebyflexibility is maintained.

Alternatively, the flexible sheet can be made of metal wire mesh,ceramic fabric or composite material. Suitable ceramic fabrics can forinstance be based on glass fibres or on ceramic fibre tape such as 3M™Nextel™. Suitable composite material can be based on carbon, Kevlar® orglass fibres embedded in polyester or epoxy resin. Using these materialsis advantageous due to their increased flexibility.

The flexible sheet when positioned for use on the opposing surface ofthe insulating end block that is primarily convex obtains a shape thatcurves in one direction only i.e. is single curved, due to the curvatureof the opposing surface of the insulating end block. It is preferredthat the curvature of the opposing surface has a radius of 0.6 to 5times the width of the cells. It is advantageous to have a large radius,as this will reduce the overall height of the assembly.

The flexible sheet when positioned for use on the opposing surface ofthe insulating end block that is primarily convex and stepped with aflat top can lead to the presence of hollow sections between theflexible sheet and the steps of the insulating end block. These hollowsections can advantageously be filled with a second insulation materialhaving better insulation properties than the insulating end block,thereby improving the overall insulation effect.

In addition the presence of stepped sides with a flat top leads to aheight of the insulating end block measured from the centre of therectangular base with the planar surface to the flat top of the opposingsurface that is less than the height obtainable with an embodiment thatis smoothly rounded outwards.

The insulating end block can be partially or completely made of anycommercially available rigid insulating material, e.g. alumina, calciumsilicate or vermiculate based blocks. Preferable insulating materialsare the calcium silicate types as they provide good machinability, arelow in weight and have low heat transfer properties and good compressivestrength.

The primarily convex surface of the insulating end block in contact withthe flexible sheet can have different geometrical shapes which canensure that the flexible sheet becomes primarily convex in form when thetwo surfaces are in contact with each other. A vertical cross-sectionthrough the insulating end block shows the primarily convex surface canfor instance be of constant radius and have the appearance of a Romanarch that is curved in shape and spanning an opening. In this embodimentthe flexible sheet becomes convex in shape, is curved in one directiononly and is smoothly rounded outwards. The primarily convex surface isnot limited to a fixed radius. Preferred is, however, a radius of 0.6 to5 times the width of the cells.

In another embodiment a vertical cross-section through the insulatingend block shows the primarily convex surface of the insulating end blockhaving one or more stepped sides and a flat top. In these embodimentsthe flexible sheet becomes primarily convex in shape and is stepwiserounded outwards. It is curved in one direction only.

In an embodiment of the invention the SOFC stack has a flexible sheetand an insulating end block with a primarily convex surface i.e. has theclamping structure of the invention at one end of the stack only. Inanother embodiment of the invention the SOFC stack has a flexible sheetand an insulating end block, with a primarily convex surface i.e., hasthe clamping structure of the invention at both ends of the stack.

FIG. 1 shows a disassembled arrangement of a conventional cross flowSOFC stack with two fuel cells. The SOFC stack comprises two solid oxidefuel cells 1 alternating with interconnects 2. The SOFC stack typicallyhas an end plate 3 made of metal or ceramics at one end and at theopposite end a manifold plate 4 typically made of metal and assisting inthe transfer of gases to fuel cells 1. When the elements of the SOFCstack are assembled, the compressive force is obtained by clamping thestack between planar end plate flanges 5 using a system of rigidtie-rods 6, springs 7 and nuts 8. In this type of assembly tie-rods 6are inserted through pipe sections 9 useful in distancing the springs 7from the fuel cell stack in order to maintain the springs 7 at a lowertemperature than the stack temperature.

FIG. 2 shows the conventional SOFC stack of FIG. 1 when assembled. Itcan be seen that pipe sections 9 ensure that springs 7 are distancedfrom the SOFC stack.

FIG. 3 shows another example of a conventional assembled SOFC stack. Inthis type of clamping structure tie-rods 6 and planar end plate flanges5 are during operation at a much lower temperature than the SOFC stack'soperation temperature. This is made possible by thermally insulating theSOFC stack at the sides of the stack by using insulation material 10.Placing additional insulation material 11 at either end of the stackadjacent to planar end plate flanges 5 allows a transfer of thecompressive force obtained during clamping through the additionalinsulation material 11. The tie-rods 6 and the planar end plate flanges5 can thus experience greater tension before undesirable creep sets in.

FIG. 4 shows the conventional disassembled SOFC stack of FIG. 3.Insulation material 10 at the sides of the stack and additionalinsulation material 11 at either end of the stack adjacent to planar endplate flanges 5 are all planar in shape.

As mentioned earlier the planar end plate flanges in these conventionalclamping structures experience a bending force when exposed to themechanical forces originating from the tie-rods 6 and the stack. Poorerelectrical contact and gas leakage therefore can occur if these bendingforces make the end plate flanges bend.

FIG. 5 shows an embodiment of the invention in which the variouscomponents of the SOFC stack clamping structure are disassembled. TheSOFC stack is inserted between two insulating end blocks 12. Eachinsulating end block 12 has a planar surface 13 adjacent to the SOFCstack and an opposing surface 14 that is the primarily convex surfaceadjacent to flexible sheet 15. The vertical cross section of theinsulating end block 12 shows the primarily convex surface having asemi-cylindrical shape and positioned on a rectangular base having thesame overall dimensions as the SOFC stack. The sides of the SOFC stackare also insulated with insulation material 10 in this embodiment.

Flexible sheet 15 is forced into the convex shape when in contact withthe insulating end block 12 and the components of the SOFC stack areassembled for clamping. The flexible sheet 15 is overall rectangular inshape with the longer sides 16 partially in contact with the insulatingend block 12 and partially in contact with insulation material 10 at thesides of the insulating end block, and the shorter sides 17 of flexiblesheet 15 extending down the sides of the SOFC stack. The shorter sides17 are bent at a pre-determined angle and length and have perforations18 for passage of the tie-rods 6. Flexible sheet 15 is curved in onedirection only and is smoothly rounded.

FIG. 6 shows the same embodiment as in FIG. 5. However, the variouscomponents of the SOFC stack clamping structure are assembled. It can beseen that after clamping the flexible sheet 15 is convex in form. Theflexible sheet 15 does therefore not have to withstand bending forcesand the mechanical tension lies in the plane of the flexible sheet. Thecompressive force is obtained after clamping with the assistance of nuts8, springs 7 and tie-rods 6 extending through perforations 18 in theflexible sheet 15.

In further embodiments of the invention, FIGS. 7 a, 7 b, 7 c, 7 d and 7e show transverse i.e. vertical cross-sections of different geometricalshapes of the insulating end block. In all cases the insulating endblock 12 has a rectangular base having a planar surface 13 and anopposing surface 14, which is primarily convex in shape and can begeometrically shaped in various manners. The different geometricalembodiments of the insulating end block 12 all ensure that the flexiblesheet 15 becomes primarily convex in shape and curves away smoothly orstepwise from the SOFC stack, curving in one direction only.

In FIG. 7 a the transverse i.e. vertical cross section through theinsulating end block 12 shows opposing surface 14 having a primarilyconvex surface that is of constant radius and has the appearance of anarch. The flexible sheet 15 is thus convex in shape, is smoothly roundedoutwards and curves in one direction only.

In FIG. 7 b the transverse i.e. vertical cross section through theinsulating end block 12 shows opposing surface 14 having a primarilyconvex surface that is of constant radius and has two steps 19 and aflat top 20 i.e. opposing surface 14 is stepped. In this embodiment theflexible sheet 15 is primarily convex in shape and is stepwise roundedoutwards, curving in one direction only. This embodiment has hollowsections 21 between the steps 19 and flexible sheet 15. The presence ofhollow sections 21 can advantageously be filled with a second insulationmaterial (not shown) having better insulation properties than insulatingend block 12, thereby improving the overall insulation effect.

In FIG. 7 c the transverse i.e. vertical cross section through theinsulating end block 12 shows opposing surface 14 having a primarilyconvex surface that has diagonally sloping sides 21 and a flat top 20.In this embodiment the flexible sheet 15 is primarily convex in shape,curves in one direction only and is stepwise rounded outwards.

The embodiments shown in FIGS. 7 b and 7 c have a flexible sheet that isstepwise rounded outwards with a flat top and curves in one directiononly. The height of the insulating end block measured from the centre ofthe rectangular base with the planar surface to the flat top of theopposing surface is less than the height obtainable with an embodimentthat is smoothly rounded outwards such as the embodiment shown in FIG. 7a. The solid oxide fuel cell stack having an insulating end block thatis stepped with a flat top thus advantageously has a lower volume andweight than a stack which is smoothly rounded outwards.

In FIG. 7 d the transverse i.e. vertical cross section through theinsulating end block 12 shows opposing surface 14 having a primarilyconvex surface that has a radius larger than the radius of the arch inFIG. 7 a. The flexible sheet is thus convex in shape, curves in onedirection only and is smoothly rounded outwards. Preferred is a radiusof 0.6 to 5 times the width of the cells.

In FIG. 7 e the transverse i.e. vertical cross-section through theinsulating end block 12 shows opposing surface 14 having a primarilyconvex surface that is triangular or pyramidal in shape with a roundedtip. In this embodiment the flexible sheet 15 also is primarily convexin shape, is curved in one direction only and is smoothly roundedoutwards.

In yet an embodiment of the invention FIGS. 8 a and 8 b show differentgeometrical shapes of the flexible sheet seen from the top of the stack.FIG. 8 a shows flexible sheet 15 and FIG. 8 b shows flexible sheet 15shown in FIG. 8 a placed on insulating end block 12. Insulating endblock 15 is convex viewed from one side of the stack and at the sametime it is convex when viewed from an angle perpendicular to the firstview. The convex shape is therefore more like a hemisphere than a halfcylinder. The convexity can be established by a combination of thegeometries shown in FIGS. 7 a to 7 e. In these embodiments the flexiblesheet 15 is curved in all directions i.e. is double curved.

1. Clamping structure for a planar solid oxide fuel cell stackcomprising a flexible sheet and a rigid, thermally insulating end block,the flexible sheet being capable of bending into a primarily convexshape, the rigid, thermally insulating end block shaped as a rectangularbase with a planar surface and an opposing surface that is primarilyconvex in shape, the flexible sheet being placed on the opposing surfaceof the rigid, thermally insulating end block, the flexible sheet therebybending to obtain a shape that is primarily convex upon compression. 2.Clamping structure according to claim 1, wherein the flexible sheet isbent to obtain a shape that is curved in one direction only.
 3. Clampingstructure according to claim 1, wherein the flexible sheet is bent toobtain a shape that is curved in all directions.
 4. Clamping structureaccording to claim 2, wherein the opposing surface of the rigid,thermally insulating end block is smoothly or stepwise rounded outwardsinto the primarily convex shape.
 5. Clamping structure according toclaim 1, wherein the flexible sheet is made of metal.
 6. Clampingstructure according to claim 5, wherein the metal is steel or an alloyof titanium, aluminium or nickel.
 7. Clamping structure according toclaim 1, wherein the flexible sheet is made of a fabric of ceramicfibres, metal wire mesh or a composite material based on glass, Kevlar®or carbon fibres embedded in polyester or epoxy resin.
 8. Clampingstructure according to claim 1, wherein the rigid, thermally insulatingend block has a thermal conductivity of 0.01-2.0 W/mK.
 9. Clampingstructure according to claim 1, wherein the rigid, thermally insulatingend block is made of alumina, calcium silicate or vermiculite basedmaterial.
 10. Clamping structure according to claim 4, wherein theopposing surface of the rigid, thermally insulating end block is steppedor pyramidal in shape.
 11. Clamping structure according to claim 1,wherein the flexible sheet has a length and a breadth of 1-2 times thecorresponding solid oxide fuel cell length and breadth in the stack. 12.Clamping structure according to claim 1, wherein the bent flexible sheethas a radius of 0.6 to 5 times the width of the solid oxide fuel cells.13. Solid oxide fuel cell stack comprising the clamping structure ofclaim 1, wherein the stack comprises one or more planar, solid oxidefuel cells interposed between end plates, at least one end plate lyingadjacent to the clamping structure comprising a flexible sheet and athermally insulating end block, the flexible sheet being capable ofbending into a primarily convex shape, the thermally insulating endblock shaped as a rectangular base with a planar surface and an opposingsurface that is primarily convex in shape, the flexible sheet beingplaced adjacent to the opposing surface of the thermally insulating endblock, the flexible sheet thereby bending to obtain a shape that isprimarily convex and the at least one end plate being in contact withthe planar surface of the rectangular base of the thermally insulatingend block.
 14. Solid oxide fuel cell stack of claim 13, wherein theclamping structure is positioned at each end plate.
 15. Process for thecompression of the solid oxide fuel cell stack of claim 13, comprisinginterposing one or more planar solid oxide fuel cells between endplates, placing adjacent to at least one end plate a clamping structurecomprising a flexible sheet and a thermally insulating end block, theflexible sheet being capable of bending into a primarily convex shape,the thermally insulating end block shaped as a rectangular base with aplanar surface and an opposing surface that is primarily convex inshape, placing the flexible sheet adjacent to the opposing surface ofthe thermally insulating end block, bending the flexible sheet to obtaina shape that is primarily convex and placing the at least one end platein contact with the planar surface of the rectangular base of thethermally insulating end block and exerting a compressive force acrosseach solid oxide fuel cell surface.
 16. Process according to claim 15,wherein the compressive force is obtained using nuts, springs andtie-rods.